The overall aim of this project is to develop novel conformal small-animal irradiator, suitable for studying genetic markers of radiation response and radiobiological models at therapeutic dose levels. There has been a revolution in functional imaging that has coincided with the development of small animal imaging devices, including microCT, microMRI, and microSPECT. Many of the institutions that have acquired these devices use them to study the efficacy of molecular markers for the diagnosis, detection, and characterization of cancer. An important component of these studies is the evaluation of response to therapeutic doses of radiation;however, the technology of animal irradiation is crude, making reproducible irradiation of in situ tumors and normal organs difficult. We have designed an image-guided small animal irradiator (microRT) that will deliver customized, conformal dose distributions to rats and mice that are highly efficient, accurately localized, with accurate dosimetry, and conformal therapy treatment planning, to support quantitative molecular imaging studies and radiobiological experiments. The system is based on a commercially available high spatial resolution orthovoltage x-ray irradiator and the radiation beam delivery will be computer controlled and automated. On-board cone-beam CT imaging will be used to aid the treatment planner in optimizing the radiation beam treatment plan. One of the sub-goals is to design the microRT unit to be highly efficient to improve throughput, so the treatment planning and delivery process will be interleaved with concurrent imaging, treatment planning, and dose delivery. Our first aim (SA1) is to build the irradiator using the existing design. The second aim (SA2) is to validate that the unit built in SA1 meets the existing specifications. The successful completion of these specific aims will open a new field of preclinical micro imagingguided radiobiology investigations on small animals.